Here are some ideas:
a) Instead of
is_category(4,5,6)
which checks if a category archive page is being displayed (see here), you can try
in_category(4,5,6)
or
in_category( array(4,5,6) )
that checks if the current post is assigned to any of the specified categories (see here).
b) You could also try
function woo_post_meta() {
if ( in_category(4,5,6) ) {
return "";
}else{
// the original woo_post_meta() code here.
}
}
c) If the original woo_post_meta()
contains an output filter, than we could add a custom filter with the above category check.
d) If you write your own child theme, you could replace the template tag:
woo_post_meta();
with
if ( !in_category(4,5,6) ) {
woo_post_meta();
}